The measurement of impedance using surface contact electrodes presents a problem in that there are variable and unknown contact impedances, which reduce the accuracy of the measurement. Contact electrode impedance measurement is for example used in impedance measurement of skin tissue (so-called bio-impedance measurement), which can be used for monitoring various conditions relating to the physical or medical state of a user of the monitoring arrangement.
The simplest setup for measuring an impedance is the so-called two-point setup, which is shown in FIG. 1.
A measurement instrument 10 has a current source 12 which is used in order to feed a known current I into the unknown impedance ZX. A voltage measurement using a voltage measurement device 14 determines the voltage drop, which is assumed to be proportional to the unknown impedance.
However, also the impedances ZW of the cables fully contribute to the voltage drop and to the measured voltage, thereby falsifying the measurement result Z:Z=U/I=I*(ZW+ZX+ZW)/I=ZW+ZX+ZW>ZX 
As a result, the measurement result Z is higher than the true impedance ZX. In order to avoid this disadvantage of the two-point impedance measurement setup, the four-point impedance measurement setup employs an additional pair of electrodes.
The four-point measurement technique is explained with reference to FIG. 2.
The current is fed through two feeding electrodes 1,4, (these can be considered as drive terminals) and the voltage drop is measured between two measurement electrodes 2,3 (these can be considered as measurement terminals). The voltage measurement is implemented with an instrument that has a very high input impedance (such as a high input impedance differential amplifier), so that almost no current flows through the measurement electrodes, i.e. their cables and contact resistance play almost no role in the measurement.
In this arrangement, one pair of electrodes is used to drive a current through the impedance to be measured, and another pair is used for the measurement, but without needing to draw significant current through the connections.
The measured voltage U is almost identical with the voltage drop UX at the unknown impedance ZX. For many applications, the four-point impedance measurement setup thus provides sufficient accuracy. However, there are still errors in the measurement resulting from the current flow through the measurement electrodes 2,3.